Controlled Hydration in Epidermal Ridges Probed by THz Time-Domain Spectroscopy

Fingerprints are common to humans, primates, and koalas but how their role in grip activities is poorly understood. Here, we reveal that the fingerprints control the hydration level of the fingertip, as required for precision gripping, by ultimately maximizing the friction. Electromagnetic waves with frequencies in the megahertz, terahertz, infrared, and visible ranges were used to identify the hydrodynamics in fingerprints, which lead to the steady-state hydration condition in `dry' and `wet' conditions. The results suggest that the fingerprint structure functions as a moisture channel for facilitating precision grip.